The invention relates to a coated element that provides accurate and simple measurement of doses of local ionizing radiation in a prescribed area of interest. The element (or dosimeter) comprises a plastic support on which is disposed a layer coated from a solution in which alanine is uniformly dispersed in a solvent-soluble elastomeric binder.
There are various processes that utilize radiationxe2x80x94e.g., sterilization, radiation therapy, food irradiation, quality checking, etc.xe2x80x94and these processes have a need to verify the radiation dose. Similarly, there is a large number of different methods to determine a dosexe2x80x94e.g., ion dosimetry (ionization in air), calorimetry (determination of heat in carbon or metals), thermoluminescence dosimetry (luminescence in solids), etc. The formation of radicals in solid organic substances on irradiation has been observed and the concentration of these radicals is proportional to the absorbed dose over a wide range. The concentration of the radicals can be determined easily by means of electron paramagnetic resonance (EPR) spectroscopy. Alanine has been widely used for this purpose due to its availability and the relative simplicity of incorporating it into practical dosimeters. An advantage of the use of organic materials such as alanine over inorganic dosimeter systems is that it can be assumed that the irradiation-induced changes in organic materials are closer to radiation effects in living tissues.
Alanine dosimetry is an accepted method to determine the radiation dose of different irradiation processes. On irradiating with ionizing radiation, radicals will be produced in alanine which are stable for long periods. This is mainly due to the inhibition of radicalxe2x80x94radical recombinations in the crystalline structure of the material that prevents the migration of large molecule fragments. The non-destructive evaluation of the radical concentration can be done using EPR spectroscopy. The determination of irradiation doses by means of EPR techniques requires a sensitive, robust and reliable instrument that can be served by a laboratory worker. A useful instrument provides such features as automated procedures for calibration and measurements. Careful adjustment of the EPR spectrometer and the selection of suitable dosimeters allows the determination of dose rates in a range from 2 Gy to 200 kGy with a total uncertainty of 3.5% (confidence level of 95%). Alanine dosimeters are small, stable, and easy to handle. They are characterized by their large measuring range and a low sensitivity to temperature and humidity. This allows for their application in radiation therapy, the irradiation of blood, as well as in industrial facilities for irradiation. The dosimeter system can be used for reference and routine dosimetry due to its high quality and low costs.
Alanine dosimeters are well known in the art. For example, in the reference: T. Kojima et al., xe2x80x9cAlanine Dosimeters Using Polymers As Bindersxe2x80x9d, Applied Radiation and Isotopes, vol. 37, No. 6, (1986), Pergamon Journals Ltd., pp. 517-520, there are numerous references to dosimeters made in pellet, rod, and film formats. Dosimeters have been made both by industrial laboratories and at academic institutions. Many of these dosimeters are in the form of molded pellets or rods. The alanine is generally blended with a synthetic or natural rubber, compounded and molded under pressure to form a variety of shapes (U.S. Pat. No. 4,668,714, J.P. 203276 J.P. 0125085, J61057-878-8). There are also references in the literature to extruded films (J01102-388-A). These extruded products, while working well, have several deficiencies. Their manufacture often requires the use of high pressures and temperatures during the molding process, requiring molding equipment that limits the sizes and shapes available. Molded dosimeters are also limited in that only moldable polymeric binders may be used. The use of molded dosimeters is also somewhat restrictive, as the size of the dosimeters tends to be very small, leading to difficulties in handling and possibly loss during irradiation.
A potential solution to these difficulties would be an alanine dosimeter coated onto a flexible support wherein the support serves not only to hold the alanine, but also provides the user with a length and width that allow easy handling. Such a coated dosimeter has been described in DE19637471 A. In this art, the alanine is coated from two, specific bindersxe2x80x94a polyoctenamer or polystyrene. Both of these binders are brittle materials and make the coating of thick alanine layers with good mechanical properties very difficult, especially when the thickness of the dosimeter layer is  greater than 100 microns. The ability to bend and shape the alanine dosimeter coated on to the plastic support can be very important in some applications, and is a significant limitation of the coated dosimeters described in the art.
It would be useful in the industry to have a dosimeter that is flexible and preferably made of material that is not brittle.
The present invention discloses an element that functions as a dosimeter, the element comprised of a thin alanine containing layer coated on a flexible plastic support. The alanine is uniformly dispersed in a solvent-soluble binder to form a coating solution and the solution used to coat a support. Hence the invention describes an element for ascertaining radiation dosage comprising: a support on which is disposed a coated layer, said coated layer comprising a binder and alanine; wherein the alanine, upon exposure to ionizing radiation, produces radicals that remain stable for long periods of time.
The present invention offers several advantages. The support is flexible and durable; avoiding the brittleness known in the prior art. The coating processes used afford the manufacturer greater control and therefore greater uniformity in the alanine content. The element can be easily handled and easily manufactured in large volume.